I agree with Laurencius regarding The Sims Medieval. The change in atmosphere, costume and lifestyle is enthralling. How you play the game itself, however, leaves a lot to be desired.
At the present time, I would not get World Adventures. Your game will want to update once you install it, and after the last update I've heard so many stories of invisible sims and disappearing inventories. It makes me afraid to take anyone on vacation. If things do get better with a future patch, it's a neat option, but since you take your sims on vacation and leave the neighborhood behind, you interact with it rather less than you would another expansion pack that you'd use during ordinary gameplay.
I'm quite liking Generations, actually. Ambitions would be my #1 favorite, Generations #2, and since you've already got Ambitions.
If you're the type to like playing with sim kids a lot, it's a must-have! If you're not I'd say Late Night might be the best choice. I don't get really into the celebrity stuff and the constant partying and dancing, but if you like playing YA and adult sims forever and ever, it can add a lot to your gameplay.
Have fun with whichever you choose!